refactor: rename Size.Height to Size.Length across codebase

"Length" is more natural than "height" for flat plate materials.
Renames the field on OpenNest.Geometry.Size, Box.Height property,
and all references across 38 files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest.Core/Part.cs

@@ -220,7 +220,7 @@ namespace OpenNest
             var part = new Part(BaseDrawing, clonedProgram,
                 location + offset,
                 new Box(BoundingBox.X + offset.X, BoundingBox.Y + offset.Y,
-                    BoundingBox.Width, BoundingBox.Height));
+                    BoundingBox.Width, BoundingBox.Length));
 
             return part;
         }